This invention relates to electrocoating a container. More particularly, the invention relates to electrocoating the interior and exterior of a metal container simultaneously.
Usually, metal containers, such as cans and the like, have their interior surfaces and exterior surfaces coated in separate operations. The interior coating is usually applied at a thicker coating weight than the subsequently applied exterior coating. The thicker interior coating is required for protection of the container from its contents and protection of the contents of the container from reaction with the metal, while the thinner exterior coating may improve handling of the container and/or its aesthetic appearance. Additionally, an exterior coating can offer protection against the container environment, such as by inhibiting the rusting of steel and tin-free steel containers in moist atmospheres and the forming of excessive oxide on aluminum containers during retort.
The coating materials may be applied by sprays, rolls, immersion or the like, using conventional polymer systems, or by using electrocoating techniques. Electrocoating can provide uniform and consistent films and thus is a desirable approach. As used herein, electrocoating is the electrodeposition of resinous coating materials, preferably organic, on electrically conductive surface areas from either anodic or cathodic electrocoating material mediums. A layer of particulate coating material is electrodeposited on an electrically charged metal substrate immersed or surrounded in the coating material as an electric current flows between the substrate and an oppositely electrically charged electrode.
U.S. Pat. No. 3,476,667, issued Nov. 4, 1969, discloses a method and apparatus for electrodepositing a coating on the inside and outside of a hollow article. An article is placed in a bath of coating material in a tank, which is equipped with electrodes, such as cathodes. An auxiliary electrode is positioned within the article and insulated therefrom. The auxiliary electrode and the tank electrodes must be of the same charge. An electrical potential is imposed between the article and all the electrode assemblies resulting in a coating on the outside of the article and a coating on the inside of the article. While such method of electrocoating may produce desirable results, it is not suitable for high speed production lines, such as are typical in the can-making industry.
Another prior art patent shows the use of inside and outside electrodes for treating the surfaces of containers. U.S. Pat. No. 2,876,358, issued Mar. 3, 1959, discloses a method and apparatus for surface treating thermoplastic containers using an outside electrode which holds a container and an inner electrode insertable into the container. The adherence characteristics of surfaces of the thermoplastic container are to be improved by subjecting the surfaces to an electrical corona discharge in an air gap between the inner and outer electrodes.
U.S. Pat. No. 3,922,213, issued Nov. 25, 1975, to the common assignee of the present invention discloses a process and apparatus for uniformly electrocoating the interior of a shaped metal container. A shaped metal container is uniformly electrocoated in an inverted position by the insertion of an electrically conducting nozzle therein through which coating material may flow into the interior of the container at a flow rate sufficient to fill the container and maintain a transient bath. An electrical potential is impressed between the container and the nozzle to coat the interior of the container. When the nozzle is removed, the container empties without further mechanical operation.
The method and apparatus of that patent considerably improved the techniques for electrocoating metal containers in high speed production lines. Slower prior art processes are avoided. The present invention is an improvement over the method and apparatus of that patent for there exists the need to electrocoat both the interior and exterior of a metal container simultaneously in one high speed operation and yet be able to apply different coating weights on the interior and exterior surfaces.